The major aim of this Six-Center Collaborative Study was to test hypotheses which implicate neurobiological mechanisms in the etiology and maintenance of the affective disorders. This collaborative approach was developed in order to provide a consortium of investigators having expertise in diverse disciplines to design and conduct the research. The study utilized uniform diagnostic criteria and behavioral methods, so as to assess the functioning of multiple biological systems in the same subjects. It provided for assessment during a baseline drug-free period and after four weeks of antidepressant drug treatment. This collaborative effort makes it possible to test earlier and current hypotheses, and to examine interactions across and among biological systems and behavior. All data analysis and the initial phase of hypothesis testing have been essentially completed. This grant application represents a request for a final period of support during which the second phase of the testing of hypotheses which involve two or more biological systems and behavior, will be brought to a conclusion.